An oxygen sensor comprising an oxygen ion-conductive solid electrolyte (e.g. stabilized zirconia) coated with a porous electrode layer (e.g. a platinum porous layer) is known. This sensor detects the theoretical A/F ratio of the exhaust gas from a burning device such as an automotive internal combustion engine in terms of the change in the electromotive force generated by the differential oxygen concentration as between the exhaust gas and the atmosphere. By means of this A/F ratio detection, the sensor will enable the engine to run at the theoretical A/F ratio.
This sensor provides a large difference in output when the A/F ratio, i.e., the weight ratio of air to fuel, is at the theoretical value of 14.7 but produces an insignificant output change when the engine is running in non-theoretical A/F regions. Therefore, the proper A/F ratio control cannot be realized by this sensor except at the theoretical A/F ratio.
Unexamined Published Japanese Patent Application No. 153155/1983 proposes an oxygen sensor using two elements, arranged parallel to each other with a gap therebetween. Each element is composed of a solid electrolyte plate having an electrode layer formed on both sides at the tip. One of the elements is used as an oxygen pump and the other element is used as an oxygen-concentration-difference-actuated electrochemical cell that detects the differential oxygen concentration as between air in the gap and exhaust gas supplied by a pump element.
The sensor disclosed in the referenced application is capable of detecting the A/F ratio of an engine over substantially the full operating range. However, it has been found that because of a V-shaped A/F ratio vs. output profile having minimum output at the theoretical A/F ratio, this sensor detects two values of A/F ratio for the same output and cannot be effectively used for the purpose of A/F ratio control unless it is definitely known whether the engine is operating in the fuel-rich or fuel-lean region.